1. Field of the Invention
The present invention relates to a digital-to-analog converter (DA converter) of the capacitive voltage divider type. The DA converter of the present invention is used, for example, for obtaining audio signals in data processing systems, such as a pulse code modulation (PCM) transmission system. A DC converter is used as a decoder in the PCM transmission system.
2. Description of the Prior Art
The prior art DA converters of the capacitive voltage type are illustrated in FIGS. 1A and 1B. In the DA converter of FIG. 1A, capacitors 41, 42 . . . 4n, each of which has, for example, a capacitance C, are connected between a common output conductor 1 and switches 51, 52, . . . 5n. One side of the selective contacts of the switches 51, 52, . . . 5n are connected to a ground conductor 2, while the other side of the selective contacts of the switches 51, 52, . . . 5n are connected to a power source conductor 3 having a potential of +V or -V.
In accordance with the selective switching of the switches 51, 52, . . . 5n for closing either side of the selective contacts, the potential of the output conductor 1 is caused to have a value which is determined by the ratio of the number of capacitors connected to the power source conductor 3 to the number of the entire capacitors. If k capacitors out of the n entire capacitors are connected to the power source conductor 3, and hence "n-k" capacitors are connected to the ground conductor 2, the potential of the output conductor 1 is equal to k/n.V, where V is the potential of the power source conductor 3. Thus a DA conversion is effected in the capacitor array circuit of FIG. 1A.
In order to discharge the charges of the output conductor 1, due to the leakage resistance of the output conductor 1 prior to an operation of the DA converter of FIG. 1A, and prevent an incorrect conversion due to said charges, the ground side contacts of the switches 51, 52, . . . 5n are closed and a switch 21 connected between the output conductor 1 and the ground conductor 2 is closed, so that the residual charges of the capacitors and the output conductor are eliminated.
Also, in order to prevent transmission of a zero output voltage, due to the closing of the switch 21, to the output of a low pass filter 9, as the output of the device of FIG. 1A, and a resulting deterioration of the signal to noise characteristic of the device of FIG. 1A, a series connected impedance conversion circuit 7 and a sample and hold circuit 8 are connected between the output of the capacitor array circuit an the low pass filter 9. The sample and hold circuit 8 samples the potentials of the output conductor 1 in asynchronism with the switching of the switch 21 and holds the sampled potentials. However, such provision of a sample and hold circuit is disadvantageous, because a sample and hold circuit is apt to effect abnormal oscillations, is difficult to manufacture, increases consumption of electrical power and increases the chip area of the integrated circuit for the device of FIG. 1A.
In the DA converter of FIG. 1B, no switch is connected between the output conductor 1 and the ground conductor 2 and hence no grounding of the output conductor 1 is effected, so that no sample and hold circuit is necessary and the output of the impedance conversion circuit is directly connected to the low pass filter 9. However, the DA converter of FIG. 1B is disadvantageous, because the potential of the output conductor is made unstable, due to the charges injected from a voltage source v (or -V) through an indefinite leakage resistance R.sub.l (or R.sub.l '). It is also disadvantageous in that the charges, which were stored in the output conductor 1 during the manufacture of the integrated circuit of the device of FIG. 1B, cause an offset of the output voltage, and the extent of this offset changes with time, the extent sometimes exceeding the limit of the operation range.
Such a prior art DA converter is disclosed in, for example, Catalogue DF 331, DF 332 of Siliconix Incorporated published in February of 1978.
The present invention has been proposed in order to prevent the above explained disadvantages in the prior art DA converters of the capacitive voltage divider type.